1. Field of the Invention
The invention relates to the field of sound wave absorption. More specifically, it relates to an anechoic and decoupling coating of an elastomeric matrix adapted to be secured to the outer surface of an underwater structure for absorbing vibrations radiating therefrom and for absorbing, with minimal reflection, waterborne sound waves directed toward the underwater structure from an external source, such as sonar. The elastomeric matrix may be a polymer such as rubber which has its hysteretic absorptive capacity enhanced by introducing time varying shear deformations in cooperation with viscous damping whereby acoustic wave energy is converted into heat.
The absorptive capability of anechoic coatings is accomplished by two mechanisms, hysteretic damping and viscous damping. An elastomeric coating, bonded to the external surface of the structure to be protected, comprises a rubber-like base material made porous by a large number of extremely small passages, and contains rubber particles, each of which hermetically seal a core filled with air, or particles of metal or dense rubber. Acoustic waves incident upon the coating deform the material which dissipates the acoustic energy via hysteretic damping, arising from a phase difference between the stress and the strain of the material. The small passages, filled with water when the coating is submerged, dissipate acoustic energy by viscous damping which occurs when water is forced through the passages as the material is deformed.
2. Brief Description of the Prior Art
Much effort has been directed in the past to materials for absorbing sound in air. The prior art is replete with panels formed of fibers and foams, all, in one form or another, defining a mass having cells, cavities or openings, or combinations thereof, adapted to absorb or otherwise hinder the passage or reflection of sound waves. Efforts were made to reduce surfaces capable of secondary radiation. It is known, for example, to provide a myriad of hollow glass spheres interspersed randomly in a mass for dissipating some sound energy by refraction as the waves travel through their surfaces to the vacuum inside. In short, masses are constructed which tend to confuse incident sound waves, and in the process, dissipate their energy for increased absorption and reduced echo. Typical examples of such materials are shown in U.S. Pat. Nos. 2,001,916; 2,036,913; 3,080,938; 3,132,714; 4,079,162; and 4,097,633.
The art of absorbing waterborne sound such as with anechoic or decoupling coatings where the material is in actual contact with the water is less developed because the need is more recent. Early methods of sound absorption were developed for establishing silent chambers for transducer testing. Many methods have been proposed for external coating for submarines to absorb probing underwater sound waves produced by sonar transducers and thereby minimize echo for preventing active detection.
It is known in the prior art, for example, to use rubber panels containing air filled cavities to dissipate acoustic waterborne energy through hysteretic loss by shear deformations. This system has as a limitation the fact that the sound absorbing and anechoic characteristics of cavity type materials such as air in rubber are affected by underwater pressure and temperature and, therefore, a material found acceptable for sound absorption at one hydrostatic pressure or temperature would be less effective at another.
It is further known to reflect sound waves to prevent their echo to a sender, to introduce materials in a coating for causing out of phase strain during an acoustic pressure cycle, or introduce viscous damping by forcing a trapped liquid to flow through a constricting orifice rather than through the more effective passages of the application, for converting acoustic energy into heat energy. Two examples of patented, or otherwise known prior art for using this technique or its equivalent absorbing waterborne sound are illustrated in FIGS. 3a and 3b. They both involve a captured liquid which, in response to incident waterborne sound waves, is caused to flow back and forth through small constricting orifices by which acoustic energy is converted into heat energy. FIG. 3a, which is a cross-sectional representation of one form of this technique, is disclosed in U.S. Pat. No. 3,647,022. The illustrated absorbing unit 1 is one of a plurality of units connected together in an array or blanket. A flexible face or membrane 113 receives incident waterborne sound waves 15 on its face resulting in induced oscillations. This imparts movement to a liquid 114 captured between the membrane 113 and a fissured cavity having small openings for allowing restricted flow. As the liquid 114 is caused to flow through these constricting orifices, a viscous loss occurs and acoustic energy is converted into heat energy.
FIG. 3b illustrates an arrangement from an article published in "Acoustica" in 1971. Oil 214 is captured between a flexible water facing diaphragm 213 on one side and a cellular rubber filled backing 215 on the other. Movement is imparted to the oil by the oscillating diaphragm, and it is forced back and forth through the small openings of a screen mesh 216, the equivalent of the orifices in FIG. 3a, for dissipation of energy.
It will be noted in the two illustrated instances of prior art, they both employ fluids being forced through constricting orifices in response to sound waves for dissipating energy. It is to be noted that the diaphragm in each instance is impervious and prevents intermixing of the contained fluid with the water in which the device is submerged. In neither case does water enter the sound absorber. In contrast, the invention of this application has capillary like passages and requires that the liquid (water), in which the matrix is submerged, enter these passages in the matrix and is caused to move to and fro in the passages by the acoustic wave for dissipation of energy. Furthermore, the constriction employed by this invention are extended passages rather than orifices or their equivalent, such as the screen in FIG. 3b. The use of the ambient fluid in the passages minimizes any impedance mismatch at the surface of the material.